1. Field of the Invention
This invention concerns conversions solutions for zinc oxide containing electrophotographic, planographic master printing plates.
2. Description of the Prior Art
Direct or off-set printing with an electrophotographic planographic master plate is a well known art which includes techniques such as lithography, flexographic printing and the like. An electrophotographic master plate is produced for printing which relies upon the difference between ink repellent, water receptive non-imaged portions of the master and water repellent, ink receptive imaged portions of the master.
The master plate is fabricated of a base material such as metal, plastic, paper or laminated base sheets to which is applied a zinc oxide-resinous binder coating having the desired photoconductive properties for the development of a latent electrostatic image by any of the well known electrostatic techniques (Xerography). Such images when developed by suitable ink-receptive, water-repellent liquid or powder developers, still require treatment of the imaged surface to enhance the desired water-receptive, ink-repellent properties of the non-imaged portions of the master surface. Enhanced water-receptive, ink-repellent characteristics are necessary for a direct or off-set master plate in order to produce fine quality copies.
A solution used for treating an imaged, developed, electrophotographic master plate (EP master) to enhance the differences between the water-receptive (hydrophilic) and water-repellent (hydrophobic) areas of the master is referred to hereinafter as an EP master conversion solution.
EP master conversion solutions have been formulated with potassium ferrocyanide usually in combination with a humectant, such as glycerine or glycol along with buffers for proper pH adjustment such as monobasic ammonium phosphate or sodium phosphate and phosphoric acid, (see U.S. Pat. No. 3,001,872). Chelating agents have been added to such ferrocyanide EP master conversion solutions (See U.S. Pat. No. 3,672,885.)
An electrophotographic master plate which employs hydrophobic imaged areas and hydrophilic non-imaged areas is commonly referred to as a lithographic printing plate. This term is used to designate plates which are adapted for either off-set or direct lithographic printing.
A conversion solution for master plates having a zinc oxide containing coating enhances the hydrophilicity of the non-imaged areas by depositing a hydrophilic zinc chelate on the non-imaged areas of the plate. An alkali metal ferrocyanide or an alkali metal ferricyanide conversion solution such as disclosed in U.S. Pat. No. 3,661,598 reacts with the nonimaged areas of the zinc oxide coating to liberate zinc ions which form an insoluble hydrophilic zinc chelate with the ferrocyanide ion which precipitate is deposited upon the non-imaged areas of the plate. Such ferrocyanide containing conversion solutions are subject to decomposition and accordingly stabilizers and buffers are often added to retard oxidative or dissociative decomposition usually caused by exposure of the solution to light or air. The possibility of decomposition is a major disadvantage of ferrocyanide based conversion solutions because of the potential for the evolution of hydrogen cyanide during decomposition at low pH levels. While buffers tend to minimize the probability of the evolution of hydrogen cyanide, the evolution of such a dangerous gas is always present with ferrocyanide based conversion solutions.